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Lasers Control Electronic Spin

Photonics.comDec 2011
CAMBRIDGE, Mass., Dec. 6, 2011 — For the first time, light has proved useful in obtaining information about the spin of electrons flowing over topological insulators — a discovery that could open up possibilities for new devices based on spintronics and magnetic data storage.

Topological insulators possess paradoxical properties: The 3-D bulk of the material behaves just like a conventional insulator — such as quartz or glass — which blocks the movement of electric currents, yet its outer surface behaves as an extremely good conductor, allowing electricity to flow freely.

Illustration of how lasers can be used to control an electric current on these new materials. Electrons (blue spheres) travel, as if on a highway, in different directions, with their axis of spin (arrows) aligned differently according to the direction of travel. A circularly polarized laser beam (left) affects electrons going in only one direction, removing them from the flow, leaving a net flow — an electric current — going the other way. (Image: Gedik Group)
The key to understanding the properties of any solid material is to analyze the behavior of electrons within the material — in particular, determining which combinations of energy, momentum and spin are possible for these electrons, said MIT assistant professor of physics Nuh Gedik. This set of combinations is what determines a material’s key properties — such as whether it is a metal or not, or whether it is transparent or opaque.

The traditional way of measuring this is to shine a light on a chunk of the solid material: The light knocks electrons out of the solid, after which their energy, momentum and spin can be measured. The challenge, Gedik said, is that such measurements just give you data for one particular point. To fill in additional points on this landscape, the traditional approach is to rotate the material slightly, take another reading, rotate it again and so on — a very slow process.

Instead, the MIT team devised a method that can provide a detailed 3-D mapping of the electron energy, momentum and spin states simultaneously. Uing short, intense pulses of circularly polarized laser light, whose time of travel can be precisely measured, they imaged how the spin and motion are related, for electrons traveling in different directions and with different momenta, all in a fraction of the time it would take using alternative methods.

They found that, instead of the spin being precisely aligned perpendicular to the direction of the electrons’ motion, there was an unexpected tilt — a sort of warping of the expected alignment — when the electrons moved with higher energies. Gedik said that understanding the distortion will prove important when the materials are used in new technologies, adding that its high-speed method of measuring electron motion and spin is not limited to studying topological insulators, but also could have applications for studying materials such as magnets and superconductors.

One unusual characteristic of the way electrons flow across the surface of these materials is that, unlike in ordinary metal conductors, impurities in the material have very little effect on the overall electrical conductivity. In most metals, impurities quickly degrade the conductivity, hindering the flow of electricity. This relative imperviousness to impurities could make topological insulators an important new material for some electronic applications, although the materials are so new that the most important applications may not yet be foreseen. One possibility is that they could be used for transmission of electrical current in situations where ordinary metals would heat up too much — because of the blocking effect of impurities — damaging the materials.

Furthermore, Gedik and his team showed that a method similar to the one they used to map the electron states also can be used to control the flow of electrons across the surface of these materials. It works because the electrons always spin in a direction nearly perpendicular to their direction of travel, but only electrons spinning in a particular direction are affected by a given circularly polarized laser beam. Thus, that beam can be used to push aside all of the electrons flowing in one direction, leaving a usable electric current flowing the other way.

Because it allows the flow of current to be controlled completely by a laser beam with no direct electronic interaction, the method could prove useful in a new kind of electromagnetic storage used in computer hard drives, which now use an electric current to “flip” each storage bit from a 0 to a 1 or vice versa. Being able to control the bits with light could offer a much quicker response time, the team believes.

This harnessing of electron behavior also could lead to the creation of spintronic circuits, using the spin of the electrons to carry information instead of their electric charge. Among other things, such devices could be an important part of creating new quantum computing systems, which many researchers think could have significant advantages over ordinary computers for solving certain kinds of highly complex problems.

The method was described by Gedik and his team in the November issue of Physical Review Letters and the December issue of Nature Nanotechnology.